Smooth muscle contraction

Question 1

-The contraction of the smooth muscles is regulated by ____________________________

Answer 1

the thick filaments (the contraction of the smooth muscles is thick filament dependent). In order to initiate contraction, the thick filaments change their conformation, following the increase of intracellular calcium.

Question 2

We can distinguish two main classes of smooth muscles:

Answer 2

1) Multi-unit smooth muscles- ciliary muscle, the iris muscle, piloerector muscles.|

2) Unitary/single/syncytial/visceral smooth muscles- gastrointestinal tract, bile ducts, ureters, uterus, and many blood vessels.

Question 3

Describe the Phasic SM

Answer 3

In some organs, the smooth muscles cells contract rhythmically (consecutive contraction-relaxation). This type of activity corresponds mainly to the single unit category, because in this case the smooth muscles cells contract in response to the action potential. The action potential reaches the muscles, the cells of this kind of smooth muscles are electrically coupled, so the action potential can travel along the muscles, and this enables the muscle fibers to contract ad relax all at the same time. E.g. the gastrointestinal tract can contract simultaneously because they are connected through gap junction. The activation is in an oscillatory pattern, so it’s not keeping the same state for a long time but it’s changing continuously.

Question 3

Characteristics of single-unit muscles?

Answer 3

this type of smooth muscles contains cells that are electrically coupled, it means that the ionic flow can move from one side to another. This kind of organization allows that when the cell is electrically stimulated the stimulation can also affect the neighboring cells (gap junctions). This coupling results in a wave of contraction (peristalsis), requiring a strict coordination of the neighboring smooth muscles cells. Often innervated by sympathetic nervous system.

Question 4

Characteristics of multi-units smooth muscles?

Answer 4

cells that compose this kind of smooth muscle are not electrically coupled. These muscles are composed of different, discrete smooth cell fibers, each of which is independent from the others. Each unit is independently innervated and commonly innervated by the sympathetic nervous system.

Question 5

Activity pattern of smooth muscles- smooth muscle types can also be distinguished according to their activation patterns:

Answer 5

Phasic SM and Tonic SM

Question 6

Several types of junctions have been identified in the smooth muscles which are ________. They serve as low resistance electrical coupling structures between the plasma membranes of two different cells

Answer 6

Gap junctions

Question 6

Smooth muscles can also operate in the opposite direction- ____________ the size of an organ by relaxing the muscles

Answer 6

increasing, the smooth muscles covering the walls of the bladder allow to increase the size of the bladder when more liquid needs to be conserved in here.

Question 7

Describe the Tonic SM

Answer 7

In other organs, some cells are tonically (continuously) activated. Their activation is not associated with action potentials , They can keep the tone (state) of contraction for a long time. E.g. vascular wall of the vessels changes (restricting or enlarging) its diameter in a way to affect the blood pressure. This modification has to be persistent over a long time period. This can be also seen in the sphincters or in the airways. The sphincter is normally contracted and can change its level of contraction but can also maintain the contraction for a long time period.

Question 8

Electrically coupled cells are _____________ or ____________

Answer 8

syncytial cells, visceral cells.

Question 9

SM cells lack the T-tubules, Instead the sarcolemma of the smooth muscles is provided with some tiny sack-like structures called _________

Answer 9

caveolae, located close to the sarcoplasmic reticulum. It’s believed that these caveolae, together with the sarcoplasmic reticulum, are involved in the regulation of the intracellular calcium concentration in these muscles

Question 9

what are adherens junctions?

Answer 9

Another kind of junction is represented by the attachment plaque, and this is mainly a mechanical coupling. It is characterized by some gaps between the plasma membranes of two cells. These structures are mainly used as anchor points for the thin filaments, so they provide a mechanical linkage.

Question 10

there are three ways that the SM decrease the amount of calcium:

Answer 10

1) Serca protein complex
2) Calcium ATPase channel
3) Sodium-Calcium antiporter

Question 11

Morphological characteristics of smooth muscle cells

Answer 11

single nucleus cells. Smaller than skeletal muscle cells, but still quite large. They are specifically larger in the region of the nucleus and are conic shaped toward the ends (sweet potato), so they are spindle shaped.

Question 11

the intracellular calcium is represented by ____________________, which can activate some calcium channels (IP3‐gated Ca2+ channel).

Answer 11

the inositol trisphosphate

Question 12

The myosin content is_______ than that of the skeletal muscles

Answer 12

lower

Question 12

Do Smooth muscles contain Troponin C?

Answer 12

No, instead they have Calmodulin with MLSK ( for myosin head phosphorylation)

Question 13

Do smooth muscles have Z lines?

Answer 13

No, instead they have dense bodies and they also lack nebulin

Question 13

The contractile activity of the smooth muscles can be controlled by different subjects, like

Answer 13

hormones, autonomic nerves (ANS), cells showing a pacemaker activity (auto-depolarization) or different drugs. All these agents allow to contract the smooth muscle by increasing the intracellular concentration of calcium

Question 14

Neural regulation of SM

Answer 14

one way to control the contraction of the muscles is the neural regulation, that depends on the type of neurotransmitters that are released and the type of nerves that are acting on the smooth muscles. It also depends on the distance from the point where the neurotransmitters are released and the place where the smooth muscles receive them, and on the receptor expressed.

Question 15

Then, this complex (called calcium-calmodulin complex) activates the______________________, which phosphorylates the regulatory light chains of myosin

Answer 15

MLCK (Myosin light-chain kinase), The phosphorylation step is crucial for the interaction of the smooth muscle myosin with the smooth muscle actin. In addition to the phosphorylation step, an ATP molecule is used to stimulate the myosin cross-bridge. After the activation (phosphorylation) of the myosin, now the head of the myosin has the ability to repeatedly bind the actin, which allows the protein to proceed to the cycling process.

Question 15

Smooth muscles are innervated by the autonomic nervous system, that is composed of two parts:

Answer 15

the sympathetic and parasympathetic systems. They move the system in opposite direction:
The sympathetic system usually increases the arousal and activates more systems.
The parasympathetic system decreases the activity.

Question 16

some smooth muscle cells that have no innervation, like the smooth muscle cells of the __________.

Answer 16

uterus

Question 16

The ___________________ can be generated by interacting with guanosine-nucleotide-binding-G protein, that activates another enzyme, the phospholipase C (PLC).

Answer 16

inositol trisphosphate, Then, the inositol reaches the sarcoplasmic reticulum and opens some channels IP3-gated calcium channel. The opening of these channels promotes the release of calcium in the sarcoplasm. The membrane potential of the cell is not affected.

Question 16

The smooth muscle in the _________ is primarily innervated by the sympathetic nervous system

Answer 16

arteries

Question 17

The sarcoplasmic calcium is increased and promotes the binding of some calcium ions to a protein called ____________

Answer 17

calmodulin

Question 18

what is the Ratchet action?

Answer 18

the result is that the thin filament is pulled toward the center of the thick filament and a force is generated. In this case the thick filament of the opposite side, after the cross-bridges cycle, is going to shorten the length of the contractile unit. This conformational change causes the release of ADP and Pi from the myosin head. After the substances are released from the myosin head another molecule of ATP can be again connected with the myosin head. This causes the head to return to the first position, where the cross-bridge was not established

Question 19

Reversal of muscle contraction requires another enzyme called _________________located in the cytosol of the muscle cells, which splits the phosphate from regulatory light chain. Then the cycling stops and contraction ceases.

Answer 19

myosin phosphatase

Question 19

what is Calcium sensitization?

Answer 19

the process that allows us to generate more force at a given calcium concentration. This is represented by the left toward shift in the calcium concentration dependence of the smooth muscle contraction. This kind of sensitization means that we need less calcium to evoke the same contraction.

Question 19

The calcium sensitization can occur in response to a decrease of the activity of ___________________

Answer 19

the MP (myosin phosphatase)

Question 19

what is RhoA signaling cascade?

Answer 19

a pathway activated by inositol trisphosphate. The activation of the RhoA stimulates the ROK (Rho kinase). The activation of the Rho kinase results in the activation of the pathway that inhibits the myosin phosphatase, thus promoting calcium sensitization

Question 20

The opposite process, the ______________________, refers to a decrease of the force contraction at a given calcium concentration

Answer 20

calcium desensitization, The calcium desensitization is evoked by activating the MP promoting a right-toward shift in the calcium dependence of the smooth muscle contraction

Question 21

Direct inhibition of MP?

Answer 21

ROK phosphorylates the myosin‐binding subunit (MBS) of MP.

Question 22

Indirect inhibition of MP?

Answer 22

ROK phosphorylates the CPI‐17which then inhibits MP.

Question 23

Catecholamines, vasopressin, endothelin, angiotensin increase the sensitivity to Ca2+ through _______________

Answer 23

RhoA/ROK signaling, thereby promoting contraction (and hence vasoconstriction).

Question 24

what is Myogenic response?

Answer 24

a physiological mechanism that helps regulate blood flow and maintain stable blood pressure within blood vessels, particularly in the small arteries and arterioles. It involves the ability of vascular smooth muscle to contract in response to changes in the stretch of the vessel wall

Question 24

what is Dihydropyridine?

Answer 24

a drug that binds the voltage-gated calcium channels and blocks them.

Question 25

what is Hydralazine?

Answer 25

involved in the opening of the potassium channels on the sarcolemma, and they lead to the relaxation of the smooth muscle because their opening results in the hyperpolarization of the cell.

Question 25

How does NO dilate the blood vessels?

Answer 25

the nitric oxide which can act as a neuromodulator within the central and peripheral nervous systems, because it relaxes the smooth muscle by increasing the cGMP concentration.

Question 26

what is Stretch Response?

Answer 26

When blood pressure increases, it causes the walls of the blood vessels to stretch. The smooth muscle cells in the vessel wall respond to this increased stretch. In response to the stretching, the smooth muscle cells contract. This contraction reduces the diameter of the blood vessel, which helps to maintain or reduce blood flow despite the increased pressure. This response protects smaller blood vessels from damage due to excessive pressure and helps ensure that organs receive a consistent blood supply, even when systemic blood pressure fluctuates.

Question 26

The rise in [Ca2+] in response to stretch involve ______________________

Answer 26

stretch‐activated channels, IP3‐signaling, and voltage‐gated Ca2+ channels.

Question 26

what is Passive tension?

Answer 26

it is the tension developed only by the elastic components of the muscle that oppose to the stretching of the muscle

Question 26

what is Active tension?

Answer 26

it is the tension developed by the muscle. Smooth muscles contain connective tissue. This tissue is mainly composed by elastin fibrils which have elastic properties. Thanks to the support of elastic fibrils, smooth muscles can tolerate high distending forces. This elastic resistance is caused by the passive components of the muscle and so they are responsible for passive length-tension